This fancy ichneumonid wasp wandered a little on the window frame, a female Tromatobia lineatoria [1] (validation determination). A new garden species.
This species lays her eggs in spider egg sacks [2].

Literature
1 Nederlands Soortenregister2 Fitton, M. G., Shaw, M. R., & Gauld, I. D., 1988. Pimpline ichneumon-flies. Hymenoptera, Ichneumonidae (Pimplinae).